290 DEDUCTION FROM THE GASEOUS THEORY OF SOLUTION. 



the laws iu the case of solutions of great conceutiation, just as tliere 

 are in the case of gases aud vapors of great concentration — for in- 

 stance, in the neighborhood of the critical point. 



T wish now to ask your attention more particularly to the actual 

 process of dissolving, and then to lay before you a hypothesis, which, 

 as it seems to me, is a logical consequence of the general tlieory. 



Imagine, then, a soluble solid in contact with water at a. fixed tern- 1 

 perature. The substance exercises a certain pressure, in right of which 

 it proceeds to dissolve. This liressure is analogous to the vapor pres- 

 sure of a volatile body in si^ace, the sjiace being represented by the 

 solvent; and the process of solution is analogous to that of vaporiza- 

 tion. As the concentration increases, the osmotic i)ressure of the dis- 

 solved portion increases, and tends to become equal to that of the un- 

 dissolved portion; just as, during vaporization in a closed space, the 

 pressure of the accumulating vapor tends to become equal to the 

 vapor pressure of the liquid. But if there be enough water present, 

 the whole of the solid will go into solution, just as the Avhole of a vola- 

 tile body will volatilize if the available space be sufficient. Such a 

 solution may be exactly saturated or unsaturated. With excess of the 

 solvent it will be unsaturated, and the dissolved matter will then be 

 in a state comparable to that of an unsaturated vapor, for its osmotic 

 pressure will be less than the possible maximum corresponding to the 

 temperature. On the other hand, if there be not excess of water pres- 

 ent during the process of solution, a condition of equilibrium aaIII be 

 arrived at when the osmotic pressure of the dissolved portion becomes 

 equal to the x)ressure of the undissolved portion, just as equilibrium 

 will be established between the volatile substance and its vapor if tlie 

 space be insufficient for complete volatilization. In such a case we get 

 a saturated solution in presence of undissolved solid, just as we nmy 

 have a saturated vapor in presence of its own liquid or solid. 



So far we have supposed the temi^erature to be stationary, but it 

 may be raised. Now, a rise of temperature will disturb equilibrium in 

 either case alike, for osmotic pressure and vapor i)ressure are both 

 increased by this means, and a re-establishment of equilibrium neces- 

 sitates increased solution or vaporization, as the case may be. 



Now, what will this constantly increasing solubility with rise of tem- 

 perature eventually lead to? Will it lead to a maximum of solubility 

 at some definite temperature beyond which increase becomes impos- 

 sible? Or will it go on in the way it has begun, so that there will 

 always be a definite, though it may be a very great, solubility for every 

 definite temperature? Or will it lead to infinite solubility before in- 

 finite temperature is obtained! One or other of these things must 

 happen, provided of course that chemical change does not intervene. 



Well, let us be guided by the analogy that has hitherto held good, ij 

 Let us see what this leads us to, and afterwards examine the availa- ~ 

 ble experimentfj-l eyid.euce, We know tUut '<\i volatile liquid, will «^t last 



